1. Field of the Invention
The present invention relates to a measuring method and apparatus, and more particularly, the present invention is suitable for use in optical heterodyne interference arrangements, for example, apparatus which measure a very small displacement of an object by using a diffraction means, such as a diffraction grating.
2. Related Background Art
Hitherto, a so-called optical encoder which measures a movement amount or a rotation amount of an object by using an optical scale has been used in the field of mechanical control. A conventional optical encoder has been disclosed in, for example, Japanese Patent Appln. Laid-Open No. 58-191907. In this optical encoder, coherent light from a light source is made to enter a diffraction grating which is a reference scale through a mirror or the like. .+-.N-th-order diffracted light emitted from this diffraction grating is reflected by a corner cube to its original direction and is also made to enter the diffraction grating. Then, two diffracted light beams of .+-.N-th order are diffracted in the same direction to interfere with each other. The intensity of the resulting interference light is detected by an optical sensor.
Since such an apparatus is small and can achieve a high resolution, it has been used for various purposes and for a variety of applications.
As machining and control have become more precise and fine, it has been required that such a measuring apparatus have a higher resolution than ever before.